The previous paragraph is only a half truth because there has been a lot of effort put into ensuring that the network does not fail. It is a little nerve-racking for the network administrator to have the responsibility of the segment fall to one router. This poses the frightening situation of a single point of failure, a term that raises the blood pressure of any of those in charge of the network. Redundancy has been built into the network with the backup designated router. Just like an understudy, the backup designated router knows all the words—in this case, the links for the segment. All routers actually have an adjacency not only with the designated router, but also with the backup designated router, which in turn has an adjacency with the designated router. If the designated router fails, the backup designated router immediately becomes the designated router.
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